6TiSCH G. Papadopoulos, Ed.
Internet-Draft N. Montavont
Intended status: Informational IMT Atlantique
Expires: January 3, 2018 P. Thubert
Cisco
July 2, 2017
Exploiting Packet Replication and Elimination in Complex Tracks in6TiSCH LLNsdraft-papadopoulos-6tisch-pre-reqs-00
Abstract
6TiSCH Packet Replication and Elimination mechanism consists in
duplicating data packets into several paths in the network to
increase reliability and provide low jitter. Over a wireless medium,
this technique can take advantage of communication overhearing, when
parallel transmissions over two adjacent paths are scheduled. This
document presents the concept and details the required changes to the
current specifications that will be necessary to enable this.
Status of This Memo
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potential collision and, consequently, it increases the end-to-end
delay performance.
This document is mainly motivated by the ongoing work in the 6TiSCH
working group. The architecture of a 6TiSCH network is detailed in
6TiSCH Architecture [I-D.ietf-6tisch-architecture] draft, which is
used for the remainder of this document. In this specification, we
focus on Complex Track with Replication and Elimination.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Tracks3.1. Tracks Overview
The 6TiSCH architecture introduces the concept of Tracks in 6TiSCH
Architecture [I-D.ietf-6tisch-architecture]. A simple track is
composed of a sequence of cells (a combination of a transmitter, a
receiver and a given channel offset) to ensure the transmission of a
single packet from a source 6TiSCH node to a destination 6TiSCH node
across a 6TiSCH multihop path.
3.2. Complex Tracks
A Complex Track is designed as a directed acyclic graph from a source
6TiSCH node towards a destination 6TiSCH node to support multi-path
forwarding, as introduced in 6TiSCH Architecture
[I-D.ietf-6tisch-architecture]. By employing DetNet Packet
Replication and Elimination (PRE) techniques, several paths may be
computed, and these paths may be more or less independent. For
example, a complex Track may branch off and rejoin over non-congruent
paths (branches).
In the following Section, we will detail Deterministic Networks PRE
techniques.
4. Packet Replication and Elimination principles
In a nutshell, PRE consists in establishing several paths in a
network to provide redundancy and parallel transmissions to bound the
delay to traverse the network. Optionnally, promiscuous listening
between paths is possible, such that the nodes on one path may
overhear transmissions along the other path. Considering the
scenario depicted in Figure 1, many different paths are possible for
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S to reach R. A simple way to take benefit from this topology could
be to use the two independent paths via nodes A, C, E and via B, D,
F. But more complex paths are possible by interleaving transmissions
from one level of the path to the upper level in a ship-in-the-night
fashion. The 6TiSCH PRE may also take advantage to the shared
properties of the medium to compensate for the potential loss that is
incurred with radio transmissions. For instance, when the source
sends to A, B may listen and get a second chance to receive the frame
without an additional transmission. Note that B would not have to
listen if it already received that particular frame at an earlier
time slot.
(A) (C) (E)
source (S) (R) (root)
(B) (D) (F)
Figure 1: A Typical Ladder Shape with Two Parallel Paths Toward the
Destination
PRE model can be implemented in both centralized and distributed
scheduling approach. In the centralized approach, a scheduler
calculates the routes and schedules the communication among the nodes
along a circuit such as a Label switched path. In the distributed
approach, each node selects its route to the destination. In both
cases, a default parent and alternate parent(s) should be selected to
set up complex tracks.
In the following Subsections, detailed description of all required
operations defined by PRE, namely, Alternative Path Selection, Packet
Replication, Packet Elimination and Promiscuous Overhearing, will be
described.
4.1. Packet Replication
The objective of PRE is to offer deterministic networking properties,
with high reliability and bounded latency. To achieve this goal,
determinism in every level of the forwarding path should be
guaranteed. By employing Packet Replication procedure, each node
transmits (i.e., replicates) each data packet to both its Default
Parent (DP) and Alternative Parent (AP). To do so, each node (i.e.,
source and intermediate 6TiSCH nodes) transmits the data packet twice
in unicast to each parent. For instance, in Figure 2, the source
6TiSCH node S is transmitting the packet to both parents, nodes A and
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B, in two different timeslots within the same TSCH slotframe. Thus,
the packet eventually obtains parallel paths to the destination.
===> (A) ====> (C) ====> (E) ====
// \\
source (S) (R) (root)
\\ //
===> (B) ====> (D) ====> (F) ====
Figure 2: Packet Replication: S transmits twice the same data packet,
to its DP (A) and to its AP (B).
4.2. Packet Elimination
The replication operation increases the traffic load in the network,
due to packet duplications. Thus, Packet Elimination operation
should be applied at each RPL DAG level to reduce the unnecessary
traffic. To this aim, once a node receives the first copy of a data
packet, it discards the following copies. Because the first copy
that reaches a node is the one that counts, and thus will be the only
copy that will be forwarded upward.
4.3. Promiscuous Overhearing
Considering that the wireless medium is broadcast by nature, any
neighbor of a transmitter may overhear a transmission. By employing
the Promiscuous Overhearing operation, DP and AP eventually have more
chances to receive the data packets. In Figure 3, when node A is
transmitting to its DP (node C), the AP (node D) and its Sibling
(node B) may decode this data packet as well. As a result, by
employing correlated paths, a node may have multiple opportunities to
receive a given data packet. This feature not only enhances the end-
to-end reliability but also it reduces the end-to-end delay.
===> (A) ====> (C) ====> (E) ====
// ^ | \\ \\
source (S) | | \\ (R) (root)
\\ | v \\ //
===> (B) ====> (D) ====> (F) ====
Figure 3: Unicast to DP with Overhearing: by employing Promiscuous
Overhearing, DP, AP and the Sibling nodes have more opportunities to
receive the same data packet.
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Internet-Draft Address Protection ND for LLN July 20175. Requirements5.1. Requirements Related to Alternative Parent Selection
To perform the Replication procedure, it is necessary to define the
Alternative Parent(s) and, consequently, the path to the destination
6TiSCH node, for each node in the 6TiSCH network. An AP can be
selected in many different ways, and is dependent on the
implementation. However, control packets should give some metrics to
discriminate between different neighbors.
Related requirements are:
Req1.1: To design such algorithm, RPL DODAG Information Object (DIO)
message format SHOULD be extended with an option to allow for a
6TiSCH node to learn additional information for its potential parent
and its list of parents.
Req1.2: The routing protocol SHOULD be extended to allow for each
6TiSCH node to select AP(s) and duplicate a packet to several next
hops.
5.2. Requirements Related to Promiscuous Overhearing
As stated previously, to further increase the 6TiSCH network
reliability and to achieve deterministic packet deliveries at the
destination 6TiSCH node, promiscuous overhearing can be considered.
As it is described in BCP 210 [RFC8180], in TSCH mode, the data
frames are transmitted in unicast mode and are acknowledged by the
receiving neighbor. To perform the promiscuous overhearing
procedure, there SHOULD be an option for the transmitted frames,
i.e., in unicast, to be overheard by the potential neighborhood
6TiSCH node.
Related requirements are:
Req2.1: The 6top Protocol [I-D.ietf-6tisch-6top-protocol] SHOULD be
extended to allow optionally a cell reservation with two receivers,
i.e., DP and AP. Considering that each frame may be transmitted
twice in unicast to each parent, then depending the transmission,
either DP will acknowledge the frame or AP will.
Req2.2: Next, to request the overhearing cells, the 6P ADD Request
Format SHOULD be transmitted either twice to each parent, i.e., DP
and AP, or once in multicast to both parents.
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Internet-Draft Address Protection ND for LLN July 20175.3. Requirements Related to Cells without ACKs
As stated in BCP 210 [RFC8180], each date frame is acknowledged by
the receiving 6TiSCH node. However, by employing promiscuous
overhearing operation, particular attention should be given to who
will acknowledge a transmission, i.e., the DP, and / or one of the
AP(s)
Related requirements are:
Req4.1: To avoid the ACK collision, the TSCH Schedule as per BCP 210
[RFC8180], only the DP MUST acknowledge the data packet.
Req4.2: Alternatively, to achieve further consistency the overheard
transmission need be acknowledged by both parents, i.e., DP and AP.
To do so, BCP 210 [RFC8180] SHOULD be extended accordingly.
5.4. Requirements Related to Packet Elimination
By employing packet replication operation, the 6TiSCH network expects
to perform the packet elimination operation along a complex Track to
bound the number of the duplicated packets, i.e., the unnecessary
traffic.
Related requirements are:
Req5.1: As per 6TiSCH Architecture [I-D.ietf-6tisch-architecture],
6TiSCH has no position about how the sequence numbers would be tagged
in the packet. However, it comes with Tagging Packets for Flow
Identification. More specifically, a 6TiSCH network expects that
timeslots corresponding to copies of a same frame along a complex
Track are correlated by configuration and, thus, does not need to
process the sequence numbers.
6. Security Considerations
TODO.
7. IANA Considerations
This document has no IANA considerations.
8. ReferencesPapadopoulos, et al. Expires January 3, 2018 [Page 7]